1. Field of Invention
The present invention relates to amplifiers in general, and more particularly, to a method and system for implementing transimpedance amplifiers with high current input while maintaining high transimpedance gain and bandwidth.
2. Description of Related Art
A transimpedance amplifier is used to convert an input current to a proportional output voltage. A typical transimpedance amplifier comprises: an input current that is supplied through a transistor input stage, the input current is typically produced by a photodiode; an output carrying an output voltage; and a coupling member connecting the input to the output. Furthermore, the input current may be small, such as 1 xcexcA, or comparatively large, such as 1 mA.
Typical uses of transimpedance amplifiers include summing currents as part of a frequency impulse response filter or processing reverse current produced by a photodiode as a function of infrared signal energy received by the photodiode.
In circumstances where a transimpedance amplifier receives high input current from a photodiode, the current may drive the transimpedance amplifier into a state of deep saturation where a large input signal spoils the sensitivity of the amplifier to a following smaller signal.
In order to prevent the transimpedance amplifier from saturation, several conventional methods have been established. FIG. 1 illustrates one embodiment of a conventional method for preventing saturation wherein a transimpedance optical receiver denoted 100 comprises: an input modulated light denoted 1, a photodiode denoted 3, a transimpedance amplifier denoted 5, a load resistor denoted 7, a non-linear diode denoted 9, and an output denoted 11.
Optical receiver 100 receives modulated light 1 from an optical fiber. Subsequently, the light falls onto photodiode 3. Moreover, photodiode 3 is connected to the inverting input of amplifier 5, resistor 7 is connected across amplifier 5, and diode 9 such as a Schottky diode is connected in parallel with resistor 7.
As shown in FIG. 1, diode 9 is added to receiver 100 in order to prevent amplifier 5 from saturation. Conventionally, a Schottky diode is chosen as diode 9 due to the fact that a Schottky diode has negligent effect on the receiver at low current input. At large current inputs, a Schottky diode conducts to limit the receiver""s output and prevents saturation.
However, traditional methods that prevent saturation such as shown in FIG. 1 are limited and cause an increase in determninistic jitter at the output. Furthermore, the bit error rate (BER) increases in response to the increased jitter and eventually reaches a point where the transferred data can no longer be recovered.
Accordingly, there is a need to prevent transimpedance amplifiers with high current input from saturation while maintaining high transimpedance gain and bandwidth.
The present invention provides a method and system for preventing a transimpedance amplifier from saturation while maintaining high transimpedance gain and bandwidth.
In a first embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. Moreover, the diode serves to divert current such that in cases where the input current is low the diode is not turned on and represents only a minimal, mostly capacitive load on the input; in cases where the input current is high, the diode conducts and diverts any excess input current from the transimpedance amplifier.
In a second embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Additionally, a resistive divider is coupled to the diode in order to further optimize the overall performance of the transimpedance amplifier.
In a third embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Furthermore, in order to alleviate switching noise generated by the excess current through the diode, an extra buffer is added in the diode shunt configuration.
In a fourth embodiment of the present invention, a diode is coupled to the input of a transimpedance amplifier in order to prevent the transimpedance amplifier from saturation. The diode serves to divert excess current from the transimpedance amplifier in cases where the input current is high. Furthermore, the configuration of the embodiment is such that the input stage may be independent of the output.
The diode shunt configuration of the present invention diverts excess current from a transimpedance amplifier in order to prevent the amplifier from saturation while maintaining high gain and bandwidth.